1. Field of the Invention
The present invention relates to an optical communication system, and more particularly to a system by directly amplitude-modulated optical transmission system in which the modulation distortion of the signal is suppressed by a feed forward control.
2. Description of the Prior Art
With development of optical cable communication networks, there have been proposed optical cable television systems (referred to as optical CATV hereinafter) in which television information and audio information are transmitted by means of optical fiber cables in place of coaxial electric cables. The use of the optical fiber cable networks can remarkably increase the amount of the information to be transmitted, enabling either increment of the number of transmission channels and extension of the transmission distance.
As the optical CATV transmission systems, there have been known a wide band frequency modulation system (referred to as FM system) and a residual side band amplitude modulation system (referred to as VSB/AM system). The VSB/AM system is disclosed in the literature entitled "FEASIBILITY OF MULTI-CHANNEL VSB/AM TRANSMISSION ON FIBER OPTIC LINK" NCTA TECHNICAL PAPER pages 17 to 25 1987. There is such a technical advantage in the FM system that S/N (signal to noise) ratio is large and the modulation distortion is small. On the other hand, technical disadvantage in the FM system is such that in order to transmit the signals in the optical fiber, it is necessary to provide a AM/FM converter for converting the electrical amplitude modulation signal into the FM signal, further it is necessary to provide a FM/AM converter for converting the FM signal received from the optical fiber into electrical AM signal. Therefore, the communication system is expensive. To the contrary, the VSB/AM system does not require such AM/FM converter and FM/AM converter and the communication system is inexpensive and simple in structure.
In the VSB/AM system, the optical signals are generated by modulating the amplitude of the current applied to a semiconductor laser. However, since the relation between the input current and the light output power of the light emitting unit such as a semiconductor laser is generally non linear, high order harmonic distortion is contained in the wave shape of the light output, resulting in lowering CN ratio which is similar to S/N ratio. Specifically, the output power of the semrconductor laser can not increase proportional to the increment of the input current to the semiconductor laser, but the output of the semiconductor contains modulation distortion proportional to the square and cube of the applied current to the semiconductor laser. Therefore, in case where the information is transmitted with the carrier whose frequency is f1 there are generated higher frequency spurious components of 2f1 and 3f1 or the like in addition to the frequency component of f1. If there is another carrier wave near the spurious frequencies 2f1 and 3f1, the modulation distortion components act to another carrier wave as the noise components, resulting in deteriorating the CN ratio. One way of preventing deterioration of the CN ratio is to drive the semiconductor laser with a constant bias current and the light generated by the semiconductor laser is modulated using amplitude modulating unit having a linear modulation property (external modulation). Another way is so called feed forward method (referred to as optical FF method hereinafter) disclosed in the literature "ANALOG TRANSMISSION OF TV-CHANNELS ON OPTICAL FIBER , WITH NON-LINEARITIES CORRECTION BY REGULATED FEED FORWARD" FRANKART. J.P. et al. REV. H.F. ELECTRON TELECOMMUNICATION VOL. 12 NO. 9 1984. In the FF method, the signals to be transmitted are applied to a semiconductor laser, and the main light signals obtained by the semiconductor laser are compared with a part of the original electric signals so as to obtain correction signals. Then a feed forward control is performed by adding the correction signals to the main received light signals after detection so that the modulation distortion generated in the semiconductor laser is suppressed.
In using the external modulation, it is one problem that it is difficult to obtain an external modulator having linear modulation property. When the optical feed forward system is produced as it is designed, it is possible to suppress the modulation distortion minimum, there must be provided two optical fiber lines always.
FIG. 1 shows a general structure of an optical communication system employing the optical feed forward system. In the optical feed forward system shown in FIG. 1 the electric analog signal Se to be transmitted is divided into two analog signals Se1 and Se2 by a branch unit 1 and one of the signals Se1 drives a main semiconductor laser 2, which generates amplitude-modulated optical signals. A part of the optical signal is derived from the optical signal by an optical branch unit 3, being converted into an electrical signal Se3 by an auxiliary light receiving element 9. Another signal Se2 divided by the electric branch unit 1 and the electric signal Se3 converted by the auxiliary light receiving element 9 have their phases and amplitudes adjusted by a delay line 6 and an amplifier 10, thereafter both signals Se2 and Se3 are subtracted by a combiner 8, whereby an electric signal Sd corresponding to the high order harmonic distortion component of the semiconductor laser 2 is formed in the combiner 8. The electric signal Sd is amplified by an amplifier 11 and thereafter drives a correction light emitting unit 12, thus correction light signal So2 is formed. The correction light signal So2 (referred to as distortion signal) is transmitted by an optical fiber line 13. The remainder optical signal Sol (referred to as main signal) taken out from the optical branch unit 3 is transmitted by an optical fiber line 4. The distortion signal So2 and the main signal So1 transmitted by the respective optical fiber lines are converted into electric signals Se4 and Se5 by the light receiving elements 5 and 14. The respective electric signals Se4 and Se5 have their phase and amplitude adjusted by a delay line 16 and an amplifier 15 and thereafter summed in a combiner 17 so as to eliminate the modulation distortion, whereby an analog signal Se without the modulation distortion can be reproduced. However, according to the optical feed forward system mentioned above, since the main signal and distortion signal are transmitted by the separate optical fiber lines, the optical fiber lines become expensive and the wide band property of the optical fiber can not be effectively used.